Foundation: Slab-on-grade or Perimeter w/ joists?

[Greg Carter]

Here's the background:

Western Maryland, 31" frost line, this will be a 20x20 shop attached to the back of my attached unheated garage. The floor will be about 1 foot above grade at the highest point, sloping to channel water away from the house.

I've been told of two main options, perimeter foundation (either poured concrete or 8" concrete blocks) with a pier in the middle supporting engineered floor joists, or a poured foundation, lined with

1" rigid insulation boards laid vertical around the perimeter, filled with rubble, 1" rigid insulation boards laid horizontal next to the vertical insulation, with a concrete pad poured over that.

This latter appeals to me because it is supposed to keep the concrete floor from being so cold in the winter (cold would have to travel past the vertical insulation and past the horizontal insulation to cool the pad)(I'm heating the shop with a safe propane heater) and there is no crawl space, vented or otherwise, to mess around with. The crawlspace would be inaccessible (not 18" clearance between dirt and joists). Don't want termites, although there's 8-12 inches from any dirt to wood, so that may not be a problem.

Shop will be for woodworking (tablesaw, bandsaw, etc.), no plumbing, one story. I'm perfectly willing to either use rubber mats where I'm standing or later come in and run 1x over 6mil sheeting, covered by tongue and groove flooring, but later, much later,hopefully.

Am I missing anything? Is one just dramatically cheaper than the other?

Thanks for any advice.

[DanG]

You may want to look into installing hydronic piping cast in the slab above the insulation. Run a small conventional water heater to heat the slab. This system is quite popular for farm shops.

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Keep the whole world singing . . . . DanG (remove the sevens) [e-mail]

[Eunoia Eigensinn]

Greg;

I'd go with the perimeter stem wall foundation, with footing below the frost line. Since it's only 4 feet high, it would be easy to make it a sandwich wall, with insulation at the core.

This eliminates the problem of having to protect the insulation (usually with labour-intensive stucco) from UV, ants, termites, chipmunks etc if placed on the exterior of the foundation walls.

One inch of insulation is pretty much useless IMO. I'd go for at least

4" or more. The cheaper beadboard (expanded polystyrene) would be fine.

I would then fill it up with 5/8" or larger crushed stone to within 8" of the top of the stem wall, install soil gas vents and put down 4 inches of rigid insulation, with a 6 mil poly sheet to function as a soil gas/moisture barrier, and cast a 4" thick slab on top, with a strip of 1" insulation around the perimeter to serve as a bond-breaker/contraction joint/thermal break.

If the walls and ceiling are reasonably well insulated, and there is moderate amount of south-facing glass, hydronic heating in the slab would be unnecessary to maintain comfortable temperatures throughout the winter.

Greg Carter wrote:

crawlspace

[default]

You can insulate a floor raised on piers just as easily as a slab laid on dirt. Which is cheaper?

[HeatMan]

You beat me to it.

Install the tubing at least. You can put in the WH later....